1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device.
2. Description of the Related Art
In recent years, an SOI (silicon on insulator) substrate has been used for a semiconductor device for high performance device. By utilizing characteristics of a thin single crystal silicon layer formed over an insulating layer, transistors formed in the integrated circuit can be electrically separated from each other completely. Further, each transistor can be formed as a fully-depleted transistor, and thus a semiconductor integrated circuit with high added value such as high integration, high speed driving, and low power consumption can be realized.
As a method for manufacturing such an SOI substrate described above, a so-called hydrogen ion implantation separation method in which hydrogen ion implantation and separation are combined is known. A typical process of a hydrogen ion implantation separation method is described below.
First, hydrogen ions are implanted into a silicon substrate to form an ion implantation layer at a predetermined depth from a surface of the substrate. Then, another silicon substrate which functions as a base substrate (a supporting substrate) is oxidized to form a silicon oxide layer. After that, the silicon substrate into which the hydrogen ions are implanted and the silicon oxide layer on the other silicon substrate functioning as the base substrate are closely attached to each other to bond the two silicon substrates together. Then, by performing heat treatment the other silicon substrate is separated at the ion implantation layer to form a thin single crystal silicon layer.
The single crystal silicon layer formed by such a method described above is very thin and has a thickness of approximately 50 nm to 30 nm in general. Therefore, use of a single crystal silicon layer formed by such a method described above is extremely suitable for a transistor for which high integration and low power consumption are needed. On the other hand, considering use for a power device or a photoelectric conversion device, a single crystal silicon layer needs to have a predetermined thickness in view of improvement of withstand voltage, improvement of photoelectric conversion efficiency, and the like.
The thickness of a single crystal silicon layer formed by an ion implantation separation method mainly depends on accelerating voltage in a step of ion implantation. Since an ion implantation layer is formed in a shallow region when accelerating voltage is reduced, the single crystal silicon layer is formed thin. On the other hand, when accelerating voltage is increased the single crystal semiconductor layer is formed thick.
According to this, it is understood that accelerating voltage is increased simply in order to thicken the single crystal semiconductor layer. However, it is not practically easy to form the thick single crystal semiconductor layer while increasing accelerating voltage. This is because, in the case of using an ion implantation apparatus suitable for mass production (an apparatus in which large current can flow), accelerating voltage cannot be more increased than a predetermined value due to its limit. It is possible to increase accelerating voltage in the case of using an apparatus with small current; however, it takes time in order to obtain a predetermined amount of ions for implantation, which is unfavorable in view of productivity. Further, in the case where ions are accelerated by high voltage exceeding 100 kV, harmful radial rays are generated; therefore, there is a problem in safety.
To solve such problems described above, a method for thickening a single crystal semiconductor layer by epitaxial growth has been considered (for example, see Reference 1).
[Reference]
[Patent Document]
[Patent Document 1]
Japanese Published Patent Application No. 2000-30995